Polyimides are known in the art for use in the manufacture of integrated circuits including chips (e.g. chip back end of line), thin film packages and printed circuit boards. Polyimides are useful in forming dielectric interlayers, passivation layers, alpha particle barriers and stress buffers. Photosensitive polyimides are particularly useful as an interlayer dielectric material to insulate the conductor wiring interconnecting the chips on a multichip module. This is known as thin-film wiring. A multichip module is an intermediate level of packaging between the chips and the circuit board. Multichip modules are generally known in the art. Multichip modules are made up of multiple layers of power, signal and ground planes which deliver power to the chips and distribute the input/output signals between chips on the module or to/from the circuit board. The use of photosensitive polyimide provides a simplified method for the fabrication of the patterned polyimide required for these thin film wiring schemes.
Photosensitive polyimides useful as a dielectric layer for conductor wiring in a multichip module are known in the art. "Polyimide Coatings" by Craig, Electronic Materials Handbook, Vol. 1, discloses negative tone photosensitive polyamic acid esters where the ester moiety is covalently bound to the polymer backbone and contains unsaturated linkages in the form of acrylates or methacrylates. Upon exposure to light, these groups, in conjunction with a suitable photopackage (photoinitiator and sensitizer), crosslink via a free radical mechanism resulting in differential solubility between the exposed and unexposed regions. Subsequent development with a suitable solvent system followed by a high temperature cure results in a negative tone imaged polyimide dielectric layer. However, during the high temperature cure, the components of the photopackage in the exposed area are volatilized resulting in excessive shrinkage of the polyimide. The shrinkage of the polyimide results in degradation of the quality of the image. In particular, the shrinkage causes tapering of the walls of image features (e.g. increase in wall angle determined by linewidth difference between the width a the bottom of the trench feature and the width at the top of the trench feature which normally expands outwardly to form a generally V-shaped trench.) Degradation of image quality can result in cross talk between circuit lines. There is still a need in the art for a suitable process for forming negative tone images of polyimide on a substrate with improved wall angles.
It is therefore an object of the present invention to provide an improved process for forming negative tone images of polyimide on a substrate.
Other objects and advantages will become apparent from the following disclosure.